The present invention relates to a system for controlling the air-fuel ratio of the air-fuel mixture for an internal combustion engine emission control system with a three-way catalyst, and more particularly to a system for controlling the air-fuel ratio of a value approximately equal to the stoichiometric air-fuel ratio for the air-fuel mixture so as to effectively operate the three-way catalyst.
Such a system as in U.S. Pat. No. 4,132,199 is a feedback control system, in which an oxygen sensor is provided to sense the oxygen content of the exhaust gases to generate an electrical signal as an indication of the air-fuel ratio of the air-fuel mixture supplied to the engine. The control system operates to compare the feedback signal from the oxygen sensor with a reference value corresponding to the stoichiometric value for producing the error signal and to control the air-fuel ratio of the mixture to be induced in the engine in accordance with the error signal.
Such a feedback control system inherently oscillates due to the detection delay of the oxygen sensor and the control delay in the system. The oscillation of the system causes the variation of the controlled air-fuel ratio relative to the stoichiometric value. Such a variation is increased during acceleration of the engine. Consequently, the emission control cannot accomplish a desired reduction of the harmful constituents of the exhaust gases.